This invention relates to hot melt thermoplastic adhesive dispensing systems and, more particularly, to systems for dispensing hot melt thermoplastic adhesive foam. Specifically, this invention is directed to a method and apparatus for controlling the density of dispensed adhesive foam in order to assure uniform foam characteristics.
Hot melt thermoplastic adhesives, or so called "hot melts", are widely used throughout industry for packaging and cartoning as well as product assembly. One of the most common problems with conventional hot melt adhesives concerns the spreadability of the adhesive after application so as to obtain sufficient surface contact between the adhesive and adhered surfaces to achieve a good bond. Generally, the greater the area of surface contact, the greater is the strength of the bond.
The relatively high viscosity and quick set time of conventional molten hot melt adhesives all combine to prevent the adhesive from spreading over a large surface area when the adhesive is applied in the molten state to a substrate. Instead of spreading, the molten hot melt adhesive sets up as a thick bead on the substrate. Even when quickly compressed, as, for example, between two flaps of a carton, the molten hot melt adhesive is difficult to spread or squeeze out to a thin film.
U.S. Pat. No. 4,059,714 entitled "Hot Melt Thermoplastic Adhesive Foam System", which is commonly assigned, discloses, among other things, that the strength of a bond achieved with a given quantity of a selected hot melt adhesive is appreciably improved if the adhesive is applied as a cellular foam rather than as a conventional non-foamed adhesive. The increased bonding strength of foamed hot melt adhesive, at least in part, results from the fact that the foamed adhesive may be spread over a substantially greater area than the same adhesive in the non-foamed state under the same amount of compression. Since the strength of the bond is a function of the surface area covered by the bond, foaming the hot melt adhesive results in a given quantity of adhesive producing a substantially stronger bond than the same adhesive non-foamed.
Foamed hot melt adhesives have also been found to have other advantages over the same adhesives applied in the non-foamed state. Specifically, foamed hot melt adhesives have been found to have a longer "open" time during which the adhesive retains its bonding strength after being applied to a substrate. The longer "open" time of the foamed hot melt adhesive over the same adhesive non-foamed results from the small air or gas cells within the foam which act as insulative barriers to prevent the escape of heat and consequent solidification of the adhesive.
Furthermore, foamed hot melt adhesives set and adhere faster than the same adhesives non-foamed when compressed between two surfaces. When the foamed hot melt adhesive is compressed between the two surfaces, the foamed adhesive spreads over a substantially greater area than the adhesive in a non-foamed state with the result that the greater surface contact causes the foamed adhesive to then give up heat faster and therefore sets more quickly than the non-foamed adhesive.
Aforementioned U.S. Pat. No. 4,059,714 discloses a method for manufacturing hot melt thermoplastic adhesive foam wherein solid thermoplastic adhesive material is melted in a heated reservoir. The molten adhesive is then mixed with air or a gas, such as nitrogen, and pressurized by either a one-stage or two-stage geared pump. Within the pump, the molten adhesive and gas are thoroughly mixed, and the gas is forced into solution with the molten adhesive by high pressure, as, for example, 300 pounds per square inch. The pressurized molten adhesive/gas solution is then supplied to a valve type adhesive dispensing gun from which the molten adhesive/gas solution is dispensed at atmospheric pressure, whereupon the gas is released from solution and becomes entrapped to form a closed cellular adhesive foam having the desirable adhesive characteristics described hereinabove. The resultant foamed adhesive in an uncompressed state sets up as a homogenous solid foam having closed air or gas cells distributed throughout the adhesive.
Typically, the use of foamed hot melt adhesive results in about a 50 percent reduction in the total quantity of hot melt adhesive required to effect the same or a better bond and without any appreciable additional material cost since the air or gas used for forming the foam is available at little or no cost. The utility of foamed hot melt adhesives is realized by a reduction in the weight of applied adhesive, which means less consumption of adhesive and less cost to the user.
Generally, hot melt thermoplastic adhesive foam dispensing apparatus is adjusted for dispensing foamed hot melt adhesive having an optimum bonding strength under the conditions in which the adhesive is used. For a given application, the particular adhesive characteristics of the foamed adhesive depend on the type of hot melt thermoplastic adhesive, the material or materials to be bonded together, the amount of adhesive to be used, the amount of compressive force to be applied, the environment in which the adhesive is dispensed, the desired "open" and set times, etc. Such factors are taken into consideration when the dispensing apparatus is adjusted so that optimum bonding strength is obtained.
The common practice has been to adjust the hot melt thermoplastic adhesive foam dispensing apparatus while packaging and cartoning or product assembly is in full operation so that optimum quantities of material are applied to the substrate and optimum bonding strength is obtained. We have discovered, however, that the bonding strength and the evenness of application of foamed hot melt adhesive is adversely affected by interruptions in the use of the dispensing apparatus. These interruptions may occur during coffee and lunch breaks, shift changes, production line shutdowns, or equipment downtime. Any reduction in bonding strength which results from these interruptions is particularly evident for a short period of time after reinitiation of packaging and cartoning or product assembly when the dispensing apparatus is again called upon to dispense foamed adhesive.
If the hot melt thermoplastic adhesive foam dispensing apparatus idles for a period of time, such as during one of these interruptions, the density of the foamed hot melt adhesive decreases, and generally so too does the bonding strength of the adhesive foam. If the dispensing apparatus is called upon to dispense foamed adhesive after it has idled for a relatively long period of time, the dispensing apparatus may even sputter and spit adhesive from the outlet nozzle and create a very uneven deposit of adhesive material on the substrate. We have discovered, and one aspect of this invention is predicated upon its discovery, that the decrease or change in foam density which occurs during one of these dispensing interruptions is attributable to the admixture of excess air or gas with the molten adhesive as the dispensing apparatus idles.